The present invention relates to a magnetic video signal recording and reproducing system (to be referred to as "VTR" in this specification) of the type wherein a plurality of rotating heads record the video signal on sequential discontinuous tracks on a magnetic tape and more particularly a magnetic recording and reproducing system which may record and reproduce the audio signal even when the tape transportation speed is considerably low.
In the prior art VTR, the video signal is recorded by a plurality of rotating magnetic heads on tracks inclined at an angle relative to the direction of transport of the magnetic tape and the audio signal is recorded by a stationary head on the edge of the magnetic tape.
The recording density of the video signal has been remarkably increased in VTR because of the improvements of the magnetic tapes, heads, techniques for processing the signal, accuracies and control techniques.
For instance, VHS VTR (capable of recording for four hours) has a recording density which is about 92 times as high as that of VTR units commonly used by broadcasting stations (of the type which uses a magnetic tape two inches in width and four heads) and about 11 times as high as that of EIAJ-I type VTR. VHS VTR uses a magnetic tape one half inch in width which is transported at the speed of 1.65 cm/sec. Extensive studies and experiments are continuing in order to further improve the characteristics of tapes and heads so that the recording density will be further increased in the future.
For instance, assume that the recording density will be doubled. Then the tape transport speed would become about 0.8 cm/sec when a magnetic tape with a half inch in width is used. At such a low transport speed, the magnetic recording and reproducing system with a stationary audio signal recording head cannot achieve high quality recording and reproduction because of the following reasons.
(a) At a low tape transport speed, the recording wavelengths become shorter so that the recording and reproduction of high frequencies becomes difficult and consequently a sufficient audio signal bandwidth (higher than 10 KHz) cannot be provided. (At a tape transport speed of 1 cm/sec, the highest frequency attainable at the present level of the audio-video techniques is 5 KHz.) PA0 (b) At a low tape transport speed, the output from a reproducing head decreases, a low S/N results and hum occurs. PA0 (c) At a low tape transport speed, the dynamic range of the level of the recorded signal becomes narrower and distortions tend to occur very frequently. PA0 (d) Accuracies of a system for transporting a magnetic tape at a low speed in a stable manner are limited so that wow and flutter are enhanced.
Because of the reasons described above, even though the video signal may be recorded at a high density the audio signal cannot be recorded at a high density.
One of the methods for obtaining a relatively high tape transport speed even when the recording density of the video signal is increased is to reduce the width of a magnetic tape to for instance 1/4 or 1/8 inches. When the recording density is same, a magnetic tape with the width of 1/4 inches must be increased in length twice as long as a magnetic tape with the width of 1/2 inches. As a result, a tape cassette may be slightly reduced in thickness but increased in surface area. (Since the wall thickness of a case, the thickness of reel hubs and the spacing between the case and the reels remain unchanged, the thickness cannot be reduced to one half but to 2/3.) As a result, a cassette loading a magnetic tape with the width of 1/4 inches would become considerably large in size as compared with a cassette containing a magnetic tape with the width of 1/2 inches when both the tapes are assumed to be capable of recording for two hours. Furthermore the 1/4 inch tape cassette would be unbalanced. With a magnetic tape with the width of 1/8 inches, a relatively high tape transport speed may be attained, but its cassette would be extremely unbalanced. In addition to the problem of shapes of cassettes, the reduction in width of magnetic tapes gives rise to the problem of skew due to the expansion and compression of a magnetic tape (that is, the discontinuity in time of the signal when the heads are switched from one to another). Furthermore the angle of inclination of the video tracks relative to the direction of travel of a magnetic tape would become small so that the recording and reproduction is easily susceptible to adverse effects by waving of a magnetic tape and consequently the interchangeability of magnetic tape cannot be ensured. Moreover a satisfactory air film would not be produced between a head and a magnetic tape so that the transport of the magnetic tape becomes unstable, resulting in jitter. Thus it is apparent that the wider the magnetic tape, the better the recording and reproduction becomes.
As described above, because of the prior art audio signal recording and reproduction systems, the recording density cannot be increased and the cassette in desired shape and size cannot be provided.
One solution to the above problems is to frequency modulate the audio signal at frequencies exterior to and lower than the bandwidth of the frequency modulated video signal, multiplex with the video signal and then record the multiplexed signal on a magnetic tape as in the case of the video disk techniques. However with this frequency multiplex system, it is impossible to record the audio signal after the video signal has been recorded. This system may be applied to an apparatus used only for reproducing signals such as a video disk, but cannot be applied to an apparatus for recording and reproducing the audio and video signals such as a VTR.